Thin-film Perovskite Solar Cells Research Articles

(Invited) Efficient Perovskite Thin Film Solar Cells on Nanostructrue

Lead halide perovskite materials, such as CH3NH3PbI3 and CH3NH3PbI3-xClx, have emerged as attractive candidates for low-cost and efficient solar cells due to their appealing optical and electrical properties. They can be readily synthesized at low temperature from earth-abundant elements thus greatly lowering the requirement on fabrication facilities. More importantly, these materials hold promise for high performance photovoltaics devices, i. e. solar cells, due to higher charge carrier mobilities and longer diffusion lengths than many organic semiconductors. In addition, their band-gap can be conveniently and widely tuned via doping process. Over the past few years, interest on perovskite photovoltaics has surged, triggered by the fast development of low-cost and efficient lead halide perovskite thin film solar cells. However, further improve performance of perovskite solar cell still remains as a challenge. Here we report fabrication of high efficiency thin film perovskite solar cells on nanostructures which demonstrate excellent flexibility and improved performance. Our systematic investigations have revealed that improved light absorption leads to device performance enhancement. Meanwhile, nanostructures also help to maintain device structure integrity, which is especially important for flexible devices.

Towards design of metal oxide free perovskite solar cell paradigm: Materials processing and enhanced device performance

The flexible perovskite thin film solar cell was fabricated using polypyrrole (PPy) as binding agent on ITO-PET flexible substrates. PPy as binding agent and PC61BM as electron transporting material on perovskite (CH3NH3PbI3)-sensitized PEDOT:PSS/ITO-PET substrates formed an efficient flexible perovskites solar cell, which demonstrated the reasonably high solar-to-electricity conversion efficiency (η) of ∼6.38% with high short circuit current (JSC) of ∼15.59mA/cm2 and open circuit voltage (VOC) of ∼0.852V under the light illumination of ∼100mW/cm2 (1.5AM). The transient photocurrent and photovoltage studies explained the improved charge transport time, efficient diffusion coefficient, diffusion length and high charge collection efficiency of the fabricated flexible perovskite solar cells. These enhancements might attribute to active PPy as binding agent along with electron transporting material of PC61BM. Thus, a reduced recombination and the improved charge transfer properties might render PPy as a cost-effective competitor in the flexible perovskites solar cell.

The efficiency limit of CH3NH3PbI3 perovskite solar cells

With the consideration of photon recycling effect, the efficiency limit of methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells is predicted by a detailed balance model. To obtain convincing predictions, both AM 1.5 spectrum of Sun and experimentally measured complex refractive index of perovskite material are employed in the detailed balance model. The roles of light trapping and angular restriction in improving the maximal output power of thin-film perovskite solar cells are also clarified. The efficiency limit of perovskite cells (without the angular restriction) is about 31%, which approaches to Shockley-Queisser limit (33%) achievable by gallium arsenide (GaAs) cells. Moreover, the Shockley-Queisser limit could be reached with a 200 nm-thick perovskite solar cell, through integrating a wavelength-dependent angular-restriction design with a textured light-trapping structure. Additionally, the influence of the trap-assisted nonradiative recombination on the device efficiency is investigated. The work is fundamentally important to high-performance perovskite photovoltaics.

Unraveling film transformations and device performance of planar perovskite solar cells

High performance (>15%) organometaltrihalide based solar cells have been demonstrated in recent years to be a promising candidate for low cost photovoltaics and have attracted significant attention in the photovoltaic community. Planar thin film perovskite solar cells, which are more easily fabricated, provide a great platform to investigate the perovskite film properties. Until now, many of the properties of perovskite thin films remain unexplored and the link between film properties and device performances is in need of investigation to further boost the efficiencies of these devices. Here, film transformation of perovskite materials is demonstrated as a critical factor to reach high performance in planar heterojunction CH3NH3PbI3−xClx solar cells. Reaction induced secondary phases can be observed and carefully controlled by tuning the processing conditions during film formation. The properties of CH3NH3PbI3−xClx films are investigated and a possible formation pathway is proposed. It is shown that the high performance devices are attainable with a small portion of secondary phases coexisting with CH3NH3PbI3 film and power conversion efficiencies of up to 14% are achieved. The correlations between the phases present, device performance and physical properties are discussed to identify the role of the secondary phases in CH3NH3PbI3−xClx material.

Optical properties and limiting photocurrent of thin-film perovskite solar cells

Thorough measurements of the optical constants of CH3NH3PbI3are used to determine the limiting parasitic losses in solar cells revealing up to 100% IQE and excellent performance at oblique incidence.

Pyramidal surface textures for light trapping and antireflection in perovskite-on-silicon tandem solar cells.

Perovskite-on-silicon tandem solar cells show potential to reach > 30% conversion efficiency, but require careful optical control. We introduce here an effective light-management scheme based on the established pyramidal texturing of crystalline silicon cells. Calculations show that conformal deposition of a thin film perovskite solar cell directly onto the textured front surface of a high efficiency silicon cell can yield front surface reflection losses as low as 0.52mA/cm(2). Combining this with a wavelength-selective intermediate reflector between the cells additionally provides effective light-trapping in the high-bandgap top cell, resulting in calculated absolute efficiency gains of 2 - 4%. This approach provides a practical and effective method to adapt existing high efficiency silicon cell designs for use in tandem cells, with conversion efficiencies approaching 35%.

A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells.

Thin-film photovoltaics based on alkylammonium lead iodide perovskite light absorbers have recently emerged as a promising low-cost solar energy harvesting technology. To date, the perovskite layer in these efficient solar cells has generally been fabricated by either vapor deposition or a two-step sequential deposition process. We report that flat, uniform thin films of this material can be deposited by a one-step, solvent-induced, fast crystallization method involving spin-coating of a DMF solution of CH3NH3PbI3 followed immediately by exposure to chlorobenzene to induce crystallization. Analysis of the devices and films revealed that the perovskite films consist of large crystalline grains with sizes up to microns. Planar heterojunction solar cells constructed with these solution-processed thin films yielded an average power conversion efficiency of 13.9±0.7% and a steady state efficiency of 13% under standard AM 1.5 conditions.

Size-controlled SiO2nanoparticles as scaffold layers in thin-film perovskite solar cells

The effects of scaffold layers composed of SiO2NPs with diameters of 15, 30, 50, 70, and 100 nm on the properties of a perovskite film were investigated . Among the various sizes of SiO2NPs, the performance of a 50 nm diameter SiO2NP based perovskite solar cell presented a high power-conversion efficiency (PCE) of 11.45%.

An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide

Organo-lead halide perovskite solar cells have emerged as one of the most promising candidates for the next generation of solar cells. To date, these perovskite thin film solar cells have exclusively employed organic hole conducting polymers which are often expensive and have low hole mobility. In a quest to explore new inorganic hole conducting materials for these perovskite-based thin film photovoltaics, we have identified copper iodide as a possible alternative. Using copper iodide, we have succeeded in achieving a promising power conversion efficiency of 6.0% with excellent photocurrent stability. The open-circuit voltage, compared to the best spiro-OMeTAD devices, remains low and is attributed to higher recombination in CuI devices as determined by impedance spectroscopy. However, impedance spectroscopy revealed that CuI exhibits 2 orders of magnitude higher electrical conductivity than spiro-OMeTAD which allows for significantly higher fill factors. Reducing the recombination in these devices could render CuI as a cost-effective competitor to spiro-OMeTAD in perovskite solar cells.